A new concept for an inexpensive and portable apparatus to perform the single breath lung diffusing capacity test (DLCO) is based on using low- cost gas sensors and a unique gas collecting system. To obtain accurate diffusion measurements with this new system, a CO sensor is required, which has a linear response that remains stable in the fact of high variations in humidity and CO concentration. Research is required to determine the feasibility of using diffusing type electrochemical CO sensors in the proposed gas collection configuration. Experiments to characterize linearity, stability and response time will be performed on diffusion type electrochemical CO sensors from four vendors. The results will determine if linearity corrections are required, and if these corrections remain constant over the life of the sensor. Following this research, a new apparatus to perform both DLCO and spirometry will be developed and marketed.